[AMDGPU][AsmParser][NFC] Get rid of custom default operand handlers.

Removes the need to add and remove them manually depending on whether
they are used in cvt*() functions. Also removes the compiler warnings
about unused handlers when it happens to be the case.

Part of <https://github.com/llvm/llvm-project/issues/62629>.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D151688